Devices and related systems exist that provide radio frequency (RF) receive (RX) functionality and RF transmit (TX) functionality within the same frequency band(s), within receive bands in which harmonics of the transmit may occur, or within closely spaced bands in the frequency spectrum. Such prior systems have used a transmit antenna and a receive antenna that are physically separate. In such devices, however, the transmit and receive functionality are not operated at the same time because when the receive circuitry and the transmit circuitry and their associated antennas are co-located within a single device or a device and its related systems, interference from the transmitter will cause degradation in the receiver performance. In other words, signals transmitted by the transmit circuitry over the transmit antenna tend to inject unwanted signals into the receive antenna and thus the receive circuitry, causing a degradation in receiver performance. For example, some portable devices and related systems have an FM transmitter and an FM receiver, use an electrically small stub antenna for the FM transmitter, and a larger antenna for the FM receiver, such as a ½ wavelength antenna in the form of a headphone wire. As stated however, such systems typically use some rudimentary method to reduce or avoid interference such as bi-modal or half-duplex approach to receive and transmit, never transmitting when attempting to receive and vice versa.
Existing devices with both RX/TX functionality and RX/TX antennas have attempted to address the interference from these simultaneous RX/TX operations in a number of ways. First, some devices operate in half-duplex or bi-modal mode, turning off the transmitter circuitry when the receiver circuitry is running and vice versa. One disadvantage of this option is that the receiver and transmitter cannot operate simultaneously, limiting the usefulness of the functionality. Second, some devices separate the transmit circuitry and antenna a large distance from the receive circuitry and antenna so that the two functions can operate simultaneously and without degradation. One disadvantage of this second option is that device/system size is increased considerably to provide a distance that is adequate to ameliorate or eliminate the degradation in performance. In addition, this second option is not practical for many portable devices, such as portable navigation devices (PNDs), handsets, and portable media players (PMPs), because the size requirements for these portable devices are too constrained to allow for adequate spacing between the receive circuitry, the transmit circuitry and their respective antennas. Third, some devices partition the frequency band to allow the transmit function to generate transmit frequencies only in channels that are far removed from the receive function and its channels. This option severely limits the number of channels in which the transmit and receive functions can operate. Finally, some devices use complicated filtering to prevent the transmit signals from affecting the receive channels. This filtering solution requires a large and costly bill of materials (BOM) and causes parts of the receive band to become unusable for transmit functions, or some parts of the receive band unusable for receive functions. This final filtering solution is thus typically costly and complicated. In short, each of these prior solutions suffers from significant disadvantages.